Diversity system



June 20, 1950 w. LYoNs DIVERSITY SYSTEM Filed Feb. 20, 1946 Ram.

klk @K NNN* W. LYONS DIVERSITY SYSTEM June 20, 1950 hours of darkness.

Patented June 20, 1950 Y *Y OFFICE DIvEnsITY SYSTEM: Walter Lymsf Bayside, N. 31', assigner ,to Radio y Corporation of ware America, a corporation of Dela- Appneauo February I2o, laiaserialfneacispss l 6 Claims,

This application relates to diversity `receiver systems in general and in particular to receivers of telegraphy or teletype signals or facsimilesignals and the like. The telegraphy, teletype-br facsimile signals may be of the on-oif (amplitude modulation) type,vorof the frequency shiftftype, and when the latter type signalsare used the-car-4 rier may be also amplitude modul-ated, `provided the frequency shift is not too great.

In certain known space diversity-systems means is provided to use signal only from that receiver wherein the best signal appears and to render the receivers which are responsive to theremaining signals, Which are not the best signals, inelfective to contribute to the output. The` improvement disclosed in this application is applicable to-di-f versity systems of this general type.

In diversity receiver stations known in the art today, there are usually two or more separate and complete diversity receiver systems operating at somewhat widely separated frequenciesfone being the frequency which practice has proven has the best day time propagation characteristic, the other being the frequency which practice has proven has the best night time ,propagation characteristic, (both choices being limited of course to .frequency ranges approved by the FCC). The receivers are alternately or simultaneouslyoperative and when the operativeness isalternative that receiver is selected for use which is` operating at the frequency which the operatorv thinksfis least'aiected by fading and similar defects'caused by multipath propagation, etc. In general it may be said that the higher frequency wave is most effective during daylight hours, and that the lower frequency wave is more eifective duringthe One of these diversity sys,- tems then is selected by the operator to supply output for a tape recorder or va teletype machine or in some cases for rebroadcast purposes.A

When the known diversity systems are used with a tape recorder both systems may be put into op-v eration and may each supply output, but only one tape message is made use of. The operator has the burden of determining which ofthe two tapes coming out of the keying machines has the least errors and of selecting that one as the signal ver? sion to be used. The operator may then shut off that diversity system which he thinks hasl the worst signal. It will be obvious that since yfading (clj'zsomsl' effects are random with respect to time, spaceand frequency, this selection by the operator may not result in use of the best signal. Moreover,itis based on the operators judgment, which may not be good. As stated above, propagation characteristicspf-radiant energy of high frequencies are better thangthose of .low `frequencies in daylight hours. .,During twilight hours the character-istics may shift from moment to moment and `it* is.` very difcultfor the- -operator, evenw-ith the two tapesv at-"hand, todecidewhich signal is best'. y Obviously,-the :poor-est. signal is vin many cases used fior varyingy lengths` of time.

e Ifga. selected- Qle ofthe diversity'receivers Oper-atingfafteletypewriter machine then -theopera-tor. ,does :notreven have the two tapes .to inf spect and compare. 'I-he'operator has Ino Way of comparing theI reception.,characteristics: of the two sys,1 :er nss. Thus he has no recourse but. to arbitrarily select `one diversity system` andV disconnect-the oth-en Y v l i l Iii-the signalisebeing rebroadcast the operator hasnowayr of ascertaining which is the best :si-gnaht except possiblyy the.roundabout way offrecording the signal or a portion thereof received by eachediversity -system'comparing the same, theneselectinga-outputfrom only the i diversity system fwhichhe considers :has the best signal.

IThe primary object of myinvention istcf-prov videA a; diversity terminal tcomprising twoor more ydiversity' receiver systems, `so-arrangedfthat outputis automatically taken'frorn that system getting Itheg best.- signale and furthermore from that receiven, of, they system selected, which.I is supply.- ing thegbestfsignal. Inother words-in my improved method ,and means frequency diversity effects are made use of as wellasspace diversi-ty effvectsr-v Then the recording apparatus or teletype machine.or-rebroadcast system willautomaticallyand continuously be-supplied with outputfromfthatrsystem operating at the frequency `most 4appropriate for use at diierent hours throughoutl the entire day.` Thel operator may then, after the propagation.characteristics have fdenitelyf changed, anda-t hisfconvenienceif he desires; :shut oif that receiver system operating ait the-frequency most affected adversely during Ap-ropag-at-ion.by multipath transmission, etc.l

Briefly-this object 'is attained by provid-ing" at .each terminal at least twosp'ace diversity ,systems'operatingtat different frequencies and so arranging i the systems that thatlrecei've'r in each system getting the best'` signal renders the other receivers ineiective, and that'systerh including the selected receiver' renders the" other systemineffectiye. y In aparticular arrangement a load'im'- pedance, common to the outputs: of the 'receivers of the `two vdiversity systems, operating at differ- .ent` frequencies best suited for cifrar-ent times ojf ;,`t`h'e daygisfso arir'ang'ed` that kthe receiver fof each partly by block diagram and partly by circuit element and circuit element connection a modication of the arrangement of Fig. 1. In Fig. 2

the received signal may compriseV an amplitude modulated carrier which is also shifted in frequency in accordance with telegraphy, facsimile, or similar signals. The amplitude modulation may be in accordance with telephone signals or telegraphy signals.

A diversity system arranged in accordance with myginvention comprises two or more diversity receiving systems operating at different frequencies, one say appropriate for daylight hours and the other appropriate for use during hours of darkness. The diversity systems operating at the two frequencies may each comprise a pluralityof receivers, at least two, three being used in the particular embodiment being described. Each diversity system makes use of three similar receivers. The receivers in one system are represented generally as channels A, B and C in Fig. 1. The channels are similar and only a part of channel A thereof is shown in detail. The other two channels B and C are shown entirely by the use of rectangles. The other diversity system comprises three similar chanels designated generally A', B' and C'. These channels operate at an input radio frequency F2, whereas the channels A, B and C operate at an input radio frequency FI.

The three channels A, B and C operate on gating tubes GTA and GTB and GTC in a manner which will be described hereinafter to make available at the output load OL, signals which are derived from that channel having output excited by the best signal. The three channels A', B' and C' likewise feed through gating tubes GT'A, GT'Band GTC to the common load OL, so that the output is further derived from that diversity system operating at the frequency Fl or F2 which is least affected by fading and is supplying the best signal.

In channel A the radio frequency amplifier converter and intermediate frequency amplifier may be of a well known type and are preferably of a type known to employees of applicants assignee as DR89. It may include in a unit 4 a radio frequency amplier, a local oscillator and a detector used for amplifying and converting the radio frequency signal which may 'be shifted in frequency in accordance with signals to a lower frequency for amplification in an intermediate frequency amplier 6. The carrier as stated above may be amplitude modulated as 'well as modulated in frequency, in which case some of the output may be selected from the IE' stages in 6 and supplied for amplification and detection and audio frequency amplification in the unit I0. The output from this unit i0 may be used as desired.

The IF output of unit 6 is fed through condenser CI to the primary winding of a transformer TI, the tuned secondary winding of which is connected to the control grid of an amplifier 'tube I2. The cathode of tube I2 is grounded through a biasing resistor and bypass condenser unit I4 to thereby complete the input circuit through the secondary winding of the transformer TI, one end of which is grounded. The anode of tube I2 is coupled to the tuned primary winding of transformer T2, the tuned secondary winding of which is coupled to the control grid of an amplifying limiter I6. This amplifying limiter has its cathode vand suppressor grid directly grounded while one end of the secondary winding of the transformer T2 is connected to ground by a biasing unit comprising a grid resistor R4 shunted by a condenser C8. The transformers TI and T2 may be of the tunable iron core type and have band pass characteristics sufficiently wide to pass without material attenuation all frequencies covering a band including mark and space frequencies, which may be separated by a frequency band of several hundred cycles.

The anode of the amplitude limiter tube IB is coupled to the tuned primary winding of a transformer T3 having a tuned secondary winding coupled differentially to the anodes of a pair of diodes D and D. The diodes may be in separate' envelopes but in the embodiment illustrated are in a single envelope. The anode of the limiting amplifier tube I6 is also coupled to the electrical center of the secondary winding of the transformer T3 by a coupling condenser CM. The cathodes of the diodes D and D are differentially coupled to the load comprising resistors RI and RII shunted by an 1F bypass condenser CIB. A point between resistors RIB and RII is coupled to the electrical center of the secondary winding of transformer T3 to complete the differential direct current circuits of the rectifier system. The coupling of the limiter to the frequency discriminator is in accordance with the teaching of Seeley, U. S. Patent #2,121,103. This discriminatorcircuit is well known in the art and serves to convert the frequency shifts in accordance with the telegraphy signals into corresponding amplitude variations which are demodulated in detectors D and D' to appear across the load including resistors RIG and RI I. If the frequency ofthe energy input to the discriminating transformer T3 is centered on the cross-over point of the sloping characteristic of the discriminator equal potentials appear at the points X and Y. If the frequency of the input to the transformer T3 goes up the potential at one of the po'mts X or Y will increase or become more positive, while the Ypotential at the other point will decrease or become less positive. Conversely, if the frequency of the input energy goes down the variation described in the preceding sentence is reversed. The signal pulses at the demodulator output are supplied to a polarity reversing switch S and from the switch S to the cathode of the channel A gate tube GTA. The purpose of the switch S is to relate the potential fed to the `cathode of the gate tube GTA to the mark and space frequencies as desired. For example, if on mark the potential at the point X rises, the potential at the point Y falls. If the switch S is lthen in the position shown the rising potential at the point X will be applied to the gate tube IGTA so that on mark frequency the potential at; the gate tube GTA becomes more positive. In practice, the mark condition may be represented Aby the higher frequency and the space Vcondition by the lower frequency. If for some reason the transmitter reverses this condition, such reversal may be corrected by changing the position of switch S. Reversal of the condition ground and. back to-.the cathod'es. mediate frequency" transformers may be tuned as shown andithe end of the secondary windings connected with the yload impedance are grounded passing condensersfl'l, l29' and 120". One of A amplifiers'are notfused in the systemas nowfar,

-respondingsfsystem ineifectivai-and Valso that system including. the..-selected receiver renders `.the other"systempineifective. Thus it will be seenthat during times when the operator is not onlyfrom. the receiver of each diversity system which has .the best Isignal but also from that re- -ceiver of both? .diversity systems which has the .best signal. whenfirstone; frequency and then the other .Thusuduring periods of twilight comes in with less fading, the operator is assured of. having fornsethe .best signal received on frequenciesfFl. and Once! theoperator is cer-l 'tainthat one' oftheffrequenciesis most effective continuously, he may, if desired, disconnect or turn olf., the .other diversity system. v

E1n the arrangement of Fig. 1, I have diversified l the systems astofrequencyshift signals only.. :Each receiver may include an amplitude modulavtion detector l0,. the outputof which may be used as desired. In the arrangement of Fig. 2, the systems'are' space and frequency diversified .with respect totheamplitude modulation of the two frequencies, and alsoas to frequency shift or modulationof .the two frequencies.

. In the system of Fig. 2 then, unit 80 may represent the radio frequency ampliers and converters ofthev unit d. ofFig; 1. This unitllthen.` includes radio frequency amplifiers, an oscillator, and a converter. An intermediate frequency `amplifier 82 `is connected with the converter. in

80 and supplies output at intermediatefrequency to the intermediate frequency amplifiers, limi-,.y

. ters, and frequency shift discriminator anddetectors 9D, and `also to amplitude modulation demodulating circuits including an IF transformer 96 Whichrsupplies 'anainplitude modulation detectorin'theform vofadiode |00. The diode vIl!!! detects the amplitude .modulation which may be .onthe carrier'energy'an'd supplies the same to a common output impedance l I0. The other chan- -nels are similarlyfconnected with-intermediate 'frequency transformers 96'- and 96;in turn .con-l nected with diodes'land IBD.

-The diodes have theirl anodes iconnected through the intermediate frequecy secondaries of the transformers to the ccmmonload' resistor HD' and to The inter-..

for energy of intermediate frequency by bytheaudio frequency Vamplifiers of one of the re-' ceivers", 'say 38," is coupled by ya coupling and blocking condenser` lZlfl to the `high potential end of resistor l lll. .'.The-remaining audio frequency ranged. 1. The' received signals may then be frequency shifted Ain accordance with telegraphy and also 4.amplitudeffmodulated in accordance with teleph- -ony or similarv signals. The modulations appeari i on vthe intermediate yfrequency -energy supplied" totrans'formelfrs 9S, 9G and 9,6" for rectification in the diodes 30, lll' and Hi8". Assume that a `strong signal comes in to transformer 96. The

negative potentialapplied to the anode of the, diode vIUE) y.due to current flowing in resistorj l" Will be lnfercome.y Re.ctil'lcation,will take place l Iand more current` will iiow in resistor l Il)v to inf crease-the negativeflpotential at the point l ist.

fngative Potential :Willbe applied: to the., f

.certainwhich frequency Fl or F2 is coming in .g fthe best, he .mayflet all. of the receivers operate ,as describedfhereinbefore to supplyoutput lnot .in the remainingdiodes. ,-the diode rectiers limits', l", H56, l and '.I". areconnected,l together and to point I3 `of the common-,resistor H0.

:an'odes'ofgallof the diodes so that only that diode getting :a ,strong signal will rectify. i When rectification takesplacethe audio frequency modulation on the carrier being rectied also appears at ,thek point. I 3l).andgis.supplied by the condenser |24 to. the. audio frequency amplifier |36, and in Athis mannerthe op'erator is using audio modulation derivedvfromv the signal appearing lin the receiver. getting the best signal.

The other diyersityreceiversystem at the right in Fig. 2 is `sirnilarlyfarranged'and-connected with the common load resistor l EB, These channels A', B and C' each comprise'aradio frequency amplifier and oscillator anddetector |40, |40 and |40".

.They alsoincludean intermediate frequency amplifier 146,1 |45' andfbl". frequencytransformers have two outputs, one

The intermediate The strongest of the six individual signals, from the six separate receiver ,channels A, B, C, A', B', and C', biases the 'anodesof fthe fiveremaining weaker-signal -K rectifiers negativegamounts such that the biases are overcome `only in that single diode receiving the strongestY signal. `Thus, rectification is inhibited in theve remaining diodes, so that that receiver, in eachof the two space diversity systenis, getting, the best signal renders the other receivers in thecorresponding system ineifective, and also that system including the selected receiver renders lthe other system ineffective.

ATheY,audioymodulations on the carrier F2 also i appear` across resistance lll) and at the input tothe audioamplifer l36. The audio amplifiers .15

rangement since the audio amplier 136 responds ISG, VIltgand l'ilneed not be used in this arntov the selected-signal irrespective of which receiver of'both groups gets the strongest signal.

In operation then `I derive signals resulting from amplitude. modulation from only that receiver ofiboth diversity systems which picks up theybestisignal, because that diode of the two systems getting the best signals blocks all of the remaining diodes.

iAs to the frequency .shift signals, the intermediate frequency amplifiers 9U, 96)' and 90" are similar to thegones described hereinbefore i in connection With Fig. 1 and including tubes `-xif, V16,;diodefsi) and D', switch S, resistances 30, .l2,-fwith the .gating control potentials produced and applied to the cathodes 35, 35' and 35'?, as described hereinbefore, to control gates GTA, GTB and GTC to supply output across ,load impedance .-OL. The intermediate frequencystages ISB, i821 and |80" also include intermediate ,frequency amplifiers, current amplitude limitera,v frequency discriminators,

frequencysshift detectors, switches S, resistance unitsv 3l, 33.', 1eltc., to operate as described hereinbefore to supply output only through that gating tube GTA or GTB or GTC to the load OL derived from the receiver getting the best signal.v Thus in this improved system of Fig. 2

I have both space. diversity and frequency diversity. and ther-effects are made use of with @asians-s respect to the amplitude-modulation on l-thlel versity systems. The-need off making ach'oice by inspectionof recordedsigrfal-s orj'byfother more round-about methods isfurl-necessary. l' 'The operator may Alea-ve --bothsystems operating i until he is sure that one of the frequencies is continuously being received in a more effective manner than the other frequency being picked up by the other system, in which case the latter may be turned off.

In both modifications the signals of varying potential appearing across OL may be used as desired to operate a teletypewriter or other recorder directly after amplification as desired, or to operate a tone keyer which operates a teletypewriter or other recorder, or they may be rebroadcast. For example, they may be used as disclosed in Schock et al. U. S, application Serial #632,9'78, filed December 5, 1945.

What is claimed is:

1. In a signalling system to be used with two carriers of different frequency each modulated in frequency by the same signal, in combination, at least two diversity receiving systems each operating at a different frequency and each including a plurality of spaced signal pickup devices, separate signal amplifying and signal demodulating means coupled to each pickup device to form separate receivers, an electronic gating device coupled to each dernodulatins means, a common output impedance coupled to all of the gating devices, means for developing a potential in each receiver related to the intensity of the signal picked up by said receiver, and means for applying the potential developed in each receiver to the gating device therefor.

2. In a signalling system to be used with two carriers of different frequency each modulated by the same signal, at least two diversity receiving systems each operating at a different one of said frequencies and each comprising a plurality of receivers having their respective antennas spaced and operating at the same frequency, a modulation detector coupled to each antenna, a common load impedance for all of said detectors, means cooperating with said common load impedance and controlled by potentials developed therein for suppressing output from all of said detectors except that one coupled to the antenna getting the best signal, and means for deriving output from said common load impedance.

3. In a signalling system to be used with two carriers of different frequency each modulated in amplitude by the same signal and in frequency by a common signal which is different from said first signal, at least two diversity receiving systems each operating ata different one of said frequencies and each comprising two or more receivers having their respective antennas spaced and operating at the same frequency, an amplitude modulation detector coupled to each antenna, a common load circuit for all of said detectors, means cooperating with said common load circuit and controlled by potentials developed therein for suppressing output from all of said detectors except that one coupled to the antenna getting the best signal, a frequency discriminator and detector coupled l'itc' antenna',- a common .ioa'diinpedanceffor all taf-said last d'etectors',and meanscooperating. with said last mentioned load fimpedance for-suppressingoutput from all of said last mentioned detectors except that one coupled to the antennaV getting the best signal.

yirfIn-'a signalling systernin combination, a

plurality of diversity receiving systemsweach comprisingI agpluralityof spaced signal pickup devices coupled to signal amplifying and signal dernodulating devices to form separate receivers,

-With the* diversity systems operating at 'l different frequencies, a common load impedance for all of the receivers of all of said diversity systems, means controlled by potentials developed in that receiver getting the best signal for supplying output to said impedance from the receiver getting the best signal, and means actuated by the potential developed across said load impedance for blocking output signal from each of the receivers except that one receiving the bestsignal.

5. In a system for receiving two different carrier currents of different frequencies modulated in amplitude in accordance with the same signal and both also shifted in frequency in accordance with another signal and for selecting for use energy of that carrier frequency which is least affected by fading during transmission, in combination, at least two diversity receiver systems each comprising a plurality of spaced4 signal pickup means, all of the signal pickup means of one system being tuned to the frequency of one of said carriers and all of the signal pickup means of the other system being tuned to the frequency of the other of said carriers, a plurality of amplitude modulation detectors in each receiver system, there being one for each signal pickup means, means for exciting each detector by current modulated in amplitude in accordance with the signals on the carrier energy in its pickup device, a common load circuit for all of said detectors, means cooperating with said common load circuit and controlled by potentials developed therein for blocking the output of all of the detectors except that detector receiving the best signal, a frequency modulation discriminator and detector for each pickup means in each system, an electronic gating device connected to each frequency modulation detector, a common load circuit for all of the gating devices, and means controlled by the intensity of the current received by the respective pickup means for turning on only that gating device connected to the frequency modulation detector for the pickup means getting the best signal.

6. Apparatus for receiving carrier currents of two different frequencies modulated in frequency in accordance with signals and for selecting for use that signal received on the strongest carrier current, comprising at least two diversity receiver systems each comprising a plurality of spaced signal pickup and amplifying means connected to form separate receivers, all of the signal pickup and amplifying means of one system being tuned to the frequency of one of said carriers and all of the signal pickup and amplifying means of the other system being tuned to the frequency of the other of said carriers, a frequency modulation discriminator and detector for each receiver in each system, an electronic gating device for each detector, a common load circuit for all of the gating devices and means controlled by the intensity of the carrier currents received by the respective re- REFERENCES CITED The following references are of record 1n the le of this patent:

UNITED STATES PATENTS Name Date Dome July 14, 1931 Number Number 12 Name Date Schroter et al Nov. 7, 1933 Beverage Feb. 9, 1937 Loughren Aug. 6, 1940 Hansell July 15, 1941 Peterson Aug. 26, 1941 Peterson July 28, 1942 Smith Feb. 2, 1943 Hollingsworth Aug. 21, 1945 Davey Sept. 11, 1945 Crosby Aug. 17, 1948 

